This invention relates to a control system of a numerically controlled fret saw machine which controls the movement of a plate shaped blank and the rotation of a fret saw blade, and more particularly a system for determining the center position of an opening adapted to receive a fret saw blade, the center position being used to form the opening through the blank with a drill.
For the sake of description, the invention will be described in connection with the manufacture of a die board utilized as a die for punching a paper board. As shown in FIG. 2, a die board is provided with a groove shaped pattern PTRN II corresponding to a desired punching or package pattern PTRN I shown in FIG. 1. A cutting blade K is implanted in a portion near PT-5 of the groove as shown in FIG. 3. At a portion J where the groove is discontinuous, the lower portion of the blade K is provided with a recess. FIGS. 4 and 5 are sectional views respectively taken along lines IV--IV and V--V in FIG. 3.
The groove shaped pattern PTRN II shown in FIG. 2 is made up of a plurality of sections PT-1, PT-2 and PT-3 separated by not cut sections PT-4, PT-5 and PT-6, respectively. More particularly, although the punching pattern PTRN I has a closed contour, the groove shaped pattern PTRN II adapted to be implanted with the cutting blade K as shown in FIG. 3 is not continuous throughout the periphery of the pattern. An arrow shown in FIG. 2 shows the course of cutting the groove. For this reason, in order to insert a fret saw blade T at a point A for starting the cutting of a groove through a section PT-3 following the not cut section PT-6 as shown in FIG. 6 which is an enlarged view of the section PT-6 shown in FIG. 2, it is necessary to position the center of a tooth of the fret saw blade at the point A and to orient the tooth in the cutting direction of the groove.
According to the prior art control system the opening HA for inserting the fret saw blade T was formed such that the cutting starting point A would come to the center of the opening. With this system, since the center of the opening coincides with the cutting starting point the position of the center of a drill can readily be determined by merely considering the spacing between the fret saw blade T and the drill. In such a case, however, the cross-sectional area of the opening HA is considerably larger than that of the fret saw blade T so that when the stamping pattern includes a number of such not cut sections or discontinuous sections, the diameter of the opening HA would cause such problems as weakening the blank and making it impossible to cut larger number of patterns out of a single blank.